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<ol>
<li><a href="https://xia-yp.gitee.io/day4/13.html">练习一</a></li>
<li><a href="https://xia-yp.gitee.io/day4/14.html">练习二</a></li>
<li><a href="https://xia-yp.gitee.io/day4/15.html">练习三</a></li>

</ol>